Design System-Compliant User Interface Generation with LLM Agents: A Comparative Study of Context Engineering Strategies

Large language model (LLM) agents are increasingly used for user interface (UI) generation, yet they often fail to apply organization-specific design systems. It remains unclear which context engineering strategy enables dependable compliance. This study compares three context engineering strategies for integrating design systems into LLM agents: instruction-based (embedding complete style guides in prompts), context-based (injecting task-relevant style guides into the context window), and registry-based (assembling pre-built components in the registry). We evaluate each strategy on compliance rate, task completion time, and token usage by generating six real-world UIs. Results show that the registry-based strategy achieved the highest compliance rate (95.08%), outperforming others with only moderate overhead. Our pilot study with nine practitioners further indicates reduced perceived difficulty and time burden while enhancing self-efficacy in their workflows. These findings demonstrate that using pre-built components in the registry enables more reliable and scalable design system-compliant UI generation with LLM agents.